A liquid crystal display device (hereinafter, abbreviated as LCD) is widely used as a compact portable display, such as a display device for personal computer. Such a portable liquid crystal display device is described, for example, in Patent Document 1. Patent Document 1 describes a display device including a first liquid crystal display portion having a first liquid crystal panel and a first case, a second liquid crystal display portion having a second liquid crystal panel and a second case, and a hinge supporting the first case and the second case pivotally. Patent Document 1 further describes a display device configured in such a manner that light from the light source disposed at the hinge portion is supplied to two openable and closable liquid crystal display portions connected to each other by the hinge portion.
Cold cathode fluorescent tubes are often used as the light source of such a liquid crystal display device. The cold cathode fluorescent tube method, however, has problems, such as the optical system including the light source becomes larger and the display performance of the liquid crystal display device is deteriorated by heat generated in the cold cathode fluorescent tube.
Further, in association with an increasing demand as a TV receiver in recent years, there has been a need for the LCD to have a further higher image quality. To this end, using light emitting diodes (hereinafter, abbreviated as LEDs) as the light source is now being discussed.
In order to avoid influences of heat generation, there is a liquid crystal display device (see, for example, Patent Document 2) configured to be illuminated by a backlight formed of a light source having, for example, a cold cathode fluorescent tube, a first light guiding body forming a planar light source and having a wedge-shaped cross section, a second light guiding body disposed at an end face portion of the first light guiding body to supply illumination light to the first light guiding body, and an optical fiber connecting the light source and the second light guiding body. Noises caused by influences of heat generation in the cold cathode fluorescent tube and application of a high frequency voltage are prevented by a configuration different from the conventional configuration, that is, by connecting the cold cathode fluorescent tube and the first light guiding body with the optical fiber as described above.
Also, there is a planar light source device using a light guiding plate having a wedge-shaped cross section, so that a reduction in size and in weight of the device can be achieved by decreasing an invalid region within the light guiding plate even when a point light source like LEDs are used (see, for example, Patent Document 3). Such a planar light source device using LEDs has satisfactory color reproducibility and is capable of obtaining a high image quality in comparison with those using cold cathode fluorescent tubes.
Further, as a planar light source device capable of making luminance across a light emitting surface homogeneous by using fewer LEDs, there is a structure in which a single optical waveguide connected to LEDs is wound and disposed on the back side of the light guiding plate (see, for example, Patent Document 4).
Also, a planar light source device achieving a higher image quality using not only LEDs for red (R) light, blue (B) light, and green (G) light, but also LEDs emitting light of other colors is now in practical use. Further, a planar light source device in which a part of LEDs are replaced with a semiconductor laser device is being discussed. This is because the semiconductor laser device has high luminance and a high output in comparison with LEDs and is therefore capable of reducing drive power and upgrading the image quality.
Meanwhile, various configurations have been developed for a multi-panel LCD forming a large screen or a multi-screen by aligning a large number of liquid crystal panels planar-wise and are now in practical use. For example, there is a multi-panel LCD having a plurality of liquid crystal display panel units disposed tile-wise and using a planar light source device formed of linearly aligned fluorescent lamps on the back side and across a plurality of the liquid crystal display panel units (see, for example, Patent Document 5).
Patent Document 1, however, fails to disclose or suggest the use of a laser as the light source. Accordingly, neither is it possible to achieve a thin-film device and space saving that can be realized by a laser capable of achieving higher luminance, nor to achieve a longer life of the battery owing to low power consumption. In addition, the configuration described in Patent Document 1 requires a light source to illuminate liquid crystal panels for each of the first liquid crystal display portion and the second liquid crystal display portion. The need for a plurality of light sources accompanied with a plurality of optical systems increases the cost and requires a further larger space.
According to the configuration described in Patent Document 2, a fluorescent tube is provided to the hinge portion connecting the liquid crystal panels. This configuration, however, has a problem that an increase in volume of the hinge portion and an increase in thickness of the liquid crystal display device itself are unavoidable with an increase of the optical system including fluorescent tubes in size.
Patent Document 3 includes a light source formed to be installed separately from the liquid crystal display device. However, it neither discloses nor suggests the configuration to reduce the size by using a plurality of liquid crystal display panels. Patent Document 4 describes the configuration used in a liquid crystal display device by realizing a homogeneous and high-luminance planar light source device using LEDs. However, it also fails to disclose or suggest the configuration to make a size compact enough to be portable using a plurality of liquid crystal display panels.
On the contrary, Patent Document 5 describes a liquid crystal display device with a large screen by combining a plurality of liquid crystal display panels into a unit. In this example, the planar light source device is formed by aligning fluorescent tubes linearly on the back side of the liquid crystal display panel units and across the liquid crystal display panel units. Accordingly, the length of the fluorescent tubes imposes a limit on the size of a large screen, and because a large number of fluorescent tubes are used, the life of the fluorescent tubes often causes irregular luminance and deterioration of luminance of the liquid crystal display device. This method is therefore thought to be difficult to ensure reliability over a long period. In addition, it is impossible to achieve a portable compact display device that requires space saving and low power consumption with this method.
Incidentally, a large screen is achieved using a single liquid crystal display panel for a TV receiver. However, in order to achieve a portable display device, it is desirable that the display device has a compact, thin-film, light, and space saving outer shape and a capability of displaying a large screen at the same time. In order to satisfy these requirements, there is required a configuration by which a display device is compact and space saving when carried along whereas the display area is expandable at the time of liquid crystal display. In order to satisfy this requirement, there is a need for a configuration that makes the liquid crystal panel space saving when carried along, for example, by folding the liquid crystal display panel. At the same time, in order to make the device itself to be compact, light, and thin-film, it is necessary to reduce the size of the optical system by using a high-luminance light source, while a low power consuming light source capable of extending the life of the battery is essential to provide the display device with mobility. For all these requirements, however, luminance of the light source is so low in the conventional configurations using LEDs and fluorescent tubes that it is impossible to achieve a compact and thin-film optical system. In addition, efficiency of fluorescent tubes and LEDs is considerably low at low power consumption including utilization efficiency of the optical system, which makes it difficult to achieve low power consumption in a case where a relatively large screen is displayed. Further, in a case where LEDs and fluorescent tubes are disposed, because light-emitting luminance and the life vary among a large number of LEDs and fluorescent tubes, there arises a problem that it is impossible to fully ensure the reliability as the liquid crystal display device as a whole.
Patent Document 1: JP-A-2002-6311
Patent Document 2: JP-A-11-167808
Patent Document 3: JP-A-2006-134661
Patent Document 4: JP-A-2006-134720
Patent Document 5: JP-A-9-500461